Typical spark-ignition linear motors of portable power tools operate at or near atmospheric pressure prior to ignition. A mixture of fuel and air is established in a combustion chamber and is ignited by a spark for combusting the mixture and driving a piston actuator of the tool. In order to achieve acceptable levels of efficiency from such motors, some sort of combustion accelerating device is added.
For example, a portion of the charge (i.e., the mix of fuel and air) is held in a pre-combustion (or primary combustion) chamber and is ignited to build sufficient pressure to spew flame jets into the main combustion (or secondary combustion) chamber. The flame jets turbulate and ignite the pre-established mix of fuel and air in the main combustion chamber.
My co-pending application Ser. No. 09/813,058 entitled Combustion Chamber System, which is hereby incorporated by reference, discloses an elongated pre-combustion chamber within which an organized flame front propels a mix of unburned fuel and air through a check valve into the main combustion chamber. The delivery of additional fuel and air into the main combustion chamber increases pressure and generates turbulence in advance of the arrival of the flame front for producing a more robust combustion in the main combustion chamber.
Although increasing power output of spark-ignited linear motors, pre-combustion chambers can present a problem when the combustion chamber needs to be scavenged and the combusted gases replaced with a fresh fuel and air mix. The pre-combustion chamber needs to be opened to circulate scavenging air. Typically, the openings between pre-combustion and main combustion chambers are small to achieve acceptable flame jet velocities, and the scavenging air must pass through the same small openings. The restriction to scavenging and subsequent recharging flows can slow cycle times and reduce scavenging efficiency.